1. Field of the Invention
The present invention generally relates to a display device, especially to an improved technology of gradation display.
2. Description of the Related Art
TFT-ELDs, namely thin-film-transistor (TFT) driven electoroluminescent displays, which include electoroluminescent elements (EL elements) driven and controlled by thin film transistors, are considered as future potential displays due to their light weight, small size, high resolution, wide visual field, low electric consumption, etc.
FIG. 4 is a circuit diagram of a conventional TFT-ELD, and FIG. 5 is a cross section of such TFT-ELD. FIG. 4 shows a unit pixel 11 of the TFT-ELD, a scanning line 12, a signal line 13, current supplying line 14, a retention capacitor 15, a selective transistor 16, a driving transistor 17, and an EL element 15. As shown in FIG. 5, the driving transistor 17 for adjusting light emission intensity (gradation) of the EL element 18 is formed on a glass substrate 10. A drain electrode of the driving transistor 17 is connected to a cathode (transparent electrode) 21 of the EL element 18, and a source electrode is connected to the analog signal supply line 14. The EL element 18 is formed of the anode 21, a luminescent layer 22, and an cathode 23. The EL element 18 may be a inorganic electroluminescent element, a low-molecular organic electroluminescent element, or a high-molecular organic electroluminescent element.
The selective transistor 16 includes a gate electrode connected to the scanning line 12, a source electrode connected to a signal line 13, and a drain electrode connected to a gate electrode of the driving transistor 17. The retention capacitor 15 is provided between the analog signal supplying line 14 and the source electrode of the selective transistor 16.
In order to cause the EL element 18 to emit light in the aforementioned structure, the scanning line 12 and the signal line 13 are set at level xe2x80x9cHxe2x80x9d, and current is conducted between the drain and the source of the selective transistor 16, whereby the driving transistor 17 is on state. An analog signal supplied from the analog supplying line 14 in this condition is delivered to the retention capacitor 15 and alters the conductance of the driving transistor 17. As a result, the EL element 18 emits light with light emission intensity pursuant to the analog signal, thereby accomplishing gradations of light emission intensity.
However, as a problem of the above-described. structure, resolution of the picture lowers due to the EL element 18 included in each pixel emitting light with unequal light emission intensity, especially in the middle gradation, because of the difference in the transistor properties of the driving transistor 17.
In order to solve this problem, the applicant of the present invention suggested in Japanese Patent Laid-Open Publication No. HEI 11-73158 a technology of displaying respective gradations by controlling on/off states of light emission of EL elements and changing the luminous area for each gradation. FIG. 6 is a circuit diagram of the TFT-ELD disclosed in said Laid-Open Publication. FIG. 6 shows an EL element included in each pixel, which is formed of EL elements 18-1 and 18-2. Such structure allows display of four gradations by controlling on/off states of EL elements 18-1 and 18-2 respectively via a 2-bit signal line formed of signal lines 13-1 and 13-2. More specifically, there are: gradation xe2x80x9c0xe2x80x9d, where neither EL element 18-1 nor 18-2 emits light; gradation xe2x80x9c1xe2x80x9d, where only EL element 18-1 emits light; gradation xe2x80x9c2xe2x80x9d, where only EL element 19-2 emits light; and gradation xe2x80x9c3xe2x80x9d, where both EL elements 18-1 and 18-2 emit light. Luminous areas of EL element 18-1 and EL element 18-2 are in a ratio of 1:2.
As shown in FIG. 7, in the structure above, signals S, D1, and D2 are respectively supplied to the scanning line 12, signal line 13-1 and signal line 13-2. When signal S is set at level xe2x80x9cHxe2x80x9d, current is conducted between the drain and the sources of selective transistors 16-1 and 16-2. In FIG. 7, gradation xe2x80x9c1xe2x80x9d is obtained when signal S is set at level xe2x80x9cHxe2x80x9d, signal D1 at level RHO, and signal D2 at level xe2x80x9cLxe2x80x9d. As a consequence, driving transistor 17-1 is turned on, and transistor 17-2 is turned off, whereby only EL element 18-1 emits light. Furthermore, in order to realize gradation xe2x80x9c2xe2x80x9d, signal S should be set at level xe2x80x9cHxe2x80x9d, signal D1 at level xe2x80x9cLxe2x80x9d, and signal D2 at level xe2x80x9cHxe2x80x9d. By doing so, driving transistor 17-2 is turned on and transistor 17-1 is turned off, and consequently, only EL element 18-2 emits light.
In this method, driving transistors 17-1 and 17-2 are to be regarded as either almost completely on state or almost completely off state. When driving transistors 17-1 and 17-2 are on state,resistance is negligibly small compared to the resistance of driving transistors 18-1 and 18-2, such that the amount of current conducted through driving transistors 17-1, 17-2, 18-1 and 18-2 depends substantially on the resistance of driving transistors 18-1 and 18-2 alone. Accordingly, light emission intensity is never uneven due to the difference in the transistor properties of driving transistors 18-1 and 18-2. Furthermore, when driving transistors 17-1 and 17-2 are off state, the voltage applied to EL elements 18-1 and 18-2 will be smaller than the threshold voltage, and driving transistors 18-1 and 19-2, will not emit light at all. Therefore, also in this case, the light emission intensity of EL elements 18-1 and 18-2 is never uneven by the difference in the transistor properties of driving transistors 18-1 and 18-2.
However, as a disadvantage of the aforementioned structure, the luminous center (the average position of the luminescent portion) shifts for each gradation and visibility is thereby decreased. Characteristics of such disadvantage will be explained with reference to FIGS. 8A-D. FIG. 8C, for example, shows a luminous center 40 of the unit pixel element 11. The EL element 18-1 shown with oblique lines means that no light is emitted, and the EL element 18-2 shown in white means that light is emitted. In FIG. 8A, the EL elements 18-1, 18-2 do not emit light. In FIG. 8B, only EL element 18-1 emits light. In FIG. 8C, only EL element 18-2 emits light. Finally, in FIG. 8D, both EL elements 18-1, 18-2 emit light. It is clear from these drawings that the position of the luminous center 40 changes for each gradation. As a consequence, when the brightness of a displayed image is changed, the position of the image shifts unfavorably. Furthermore, if the displayed image is actually observed here, the displayed image will be seen to flicker, causing an impression of unnatural display or fatigue to the viewer.
Accordingly, the object of the present invention is to overcome such disadvantage and to provide a display device wherein a luminous center does not shift for each light emission gradation.
In the present invention, in order to achieve said object, a unit pixel is formed of multiple EL elements whose luminescent portions corresponding to each gradation are arranged point-symmetrically with one another with respect to a prescribed point. Such structure allows provision of a display device wherein the position of a luminous center does not change for each gradation. xe2x80x9cPrescribed positionxe2x80x9d here means, for example, a luminous center of the EL element upon realizing the gradation of minimum luminance.
Furthermore, each electroluminescent element is preferably configured to have a state of xe2x80x9cemissionxe2x80x9d or xe2x80x9cnon-emissionxe2x80x9d. By controlling on/off of the multiple Aluminescent elements, it is possible to prevent uneven aluminance caused by difference in the properties of luminescent elements. In order to achieve the structure above, electroluminescent elements may, for example, be used as luminescent elements, so that thin-film transistors may control the on/off states of light emission by the luminescent elements.